The present invention relates to a flexible pipe for transporting, over long distances, a fluid which is under pressure and possibly at a high temperature, such as a gas, petroleum, water or other fluids. The invention relates most particularly to a pipe intended for offshore oil exploration. It is known that there are, on the one hand, bottom pipes, called flow lines, that is to say flexible pipes which are unwound from a barge in order to be laid generally on the bottom of the sea and connected to the subsea installations, such pipes working mainly in static mode, and, on the other hand, rising columns, called risers, that is to say flexible pipes which are unwound from a surface installation such as a platform and are connected to the subsea installations and most of which do not lie on or below the seabed, such pipes working essentially in dynamic mode. The invention relates more particularly to pipes working in dynamic mode.
The flexible pipes used offshore must be able to resist high internal pressures and/or external pressures and also withstand longitudinal bending or twisting without the risk of being ruptured.
They have various configurations depending on their precise use but in general they satisfy the constructional criteria defined in particular in the standards API 17 B and API 17 J drawn up by the American Petroleum Institute under the title “Recommended Practice for Flexible Pipe”.
A flexible pipe generally comprises, from the inside outwards:                an internal sealing sheath made of a plastic, generally a polymer, resistant to a greater or lesser extent to the chemical action of the fluid to be transported;        a pressure vault resistant mainly to the pressure developed by the fluid in the sealing sheath and consisting of the winding of one or more interlocked metal profile wires (which may or may not be self-interlockable) wound in a helix with a short pitch around the internal sheath;        at least one ply (and generally at least two crossed plies) of tensile armour layers whose lay angle measured along the longitudinal axis of the pipe is less than 55° C.; and        an external protective sealing sheath made of a polymer.        
Such a structure is that of a pipe with a so-called smooth bore. In a pipe with a so-called rough bore, a carcass consisting of an interlocked metal strip is also provided inside the internal sealing sheath, serving to prevent the pipe collapsing under external pressure.
The pressure vault consists of a winding of non-touching turns so as to give the pipe a degree of flexibility. The expression “non-touching turns” is understood to mean turns between which a certain space or interstice, called hereafter “gap”, is left, which gap may be greater the larger the wound profile wire.
There are two types of pressure vault:                pressure vaults for static applications;        pressure vaults for dynamic applications.        
For static applications, the pressure vaults only have to withstand the internal and external pressures. They are not subjected to the fatigue caused by the rubbing due to dynamic stressing. In this case, vault wires with any type of interlocking may be used. The wire (cross section and inertia) is selected according to the internal and external pressures.
For dynamic applications, such as those mainly intended by the invention, the pressure vaults must withstand, in addition to the internal and external pressures, large stresses due to dynamic stressing. These stresses are due to contacts between the wires constituting the vault. These contacts cause rubbing which, combined with the large bearing pressures, result in a reduction in the lifetime of the profile wires by fatigue cracking.
This problem is encountered, for example in the pipe known from U.S. Pat. No. 4,549,581 which shows the interlocking by U-shaped fasteners of the U-shaped wires from below. These fasteners are too weak to withstand the dynamic stressing. In fact, such a fastener withstands only little stress by itself; it bends and bears on the outline of the profile wire. This results in bending moments and therefore alternating stresses, of greater or less magnitude according to the position and therefore of the level of dynamic stressing. In this case, the behaviour under dynamic conditions depends on the initial position of the profile wire with respect to the fastener and on the level of dynamic stressing, and therefore on the magnitude of the waves. The cross sections and the characteristics of the material must be adjusted according to the levels of severity. This type of interlocking considerably limits the performance of the pressure vault in dynamic applications.
This is why the Applicant has already proposed in document FR 2 727 738 to raise the fastening region into the upper portion of the pressure vault. Since the interlocking is carried out from above, the fastener is little exposed to the internal pressure (which is taken up by the profile, in this case “teta”, wire) and to the amplitudes of displacement due to the dynamic stressing.
According to document EP 0 431 142, which also relates to a pressure vault with a “teta” profile wire, the interlocking is done approximately level with the neutral fibre.
In Patent U.S. Pat. No. 5,730,188, it has been proposed, in order to improve the fatigue resistance in dynamic mode of “zêta” wires, to raise the interlocking region into the upper portion of the vault and to make a chamfer on the upper flange of the wire. This profile wire makes it possible to limit contacts in the interlocking region and thus increases the fatigue resistance of the pressure vault.
The Applicant has also proposed in document FR 2 783 142 to use a lightened profile wire of large moments of inertia, in the form of an I, which, for dynamic applications, is also interlocked from above.
It may therefore be seen that, for dynamic applications, a consensus has been established whereby the profile wire is interlocked exclusively from above.
For deepsea applications, it is sought to have moment of inertia at the pressure vault so as also to withstand the external pressure. In this case, the profile wire constituting the vault has a relatively large height which, in a dynamic application with interlocking from above, creates a large gap. The internal sheath may creep into this large volume due to the effect of temperature and of the internal pressure, which results either in damage to the sheath or in disruption to the interlocking of the pressure vault. This problem may be alleviated by providing the pressure sheath with additional thickness, but that increases the cost of this sheath.
This is why the Applicant has also proposed, in document FR 2 782 141, anti-creep devices to mask these gaps. However, the use of these devices increases the cost of manufacturing these pressure vaults.